This application claims benefit of UK application Number GB 0200113.0 filed on Jan. 4, 2002, by the same inventors.
This invention is concerned with a method for distinguishing between rocks laid down in different depositional environments. It is of particular utility in exploration for oil and gas and represents an extension of the invention described and claimed by the same inventors in U.S. Pat. No. 5,999,485, which is hereby incorporated herein.
When a borehole is drilled in the search for oil and gas, it is common practice to lower measuring instruments (xe2x80x9clogging toolsxe2x80x9d) into the hole, and to record the observed variations (xe2x80x9clogsxe2x80x9d) of some physical property of the rocks (such as density, or acoustic velocity, or radioactivity) as a function of depth in the hole. The interpretation of such logs, and the making of inferences about the geology penetrated by the borehole, is an advanced art. However, other advances are desirable, and one matter where advance is still required is in the use of the logs to define the depositional environment in which the rocks were laid down. In particular, additional methods are required to distinguish between rocks laid down where the rate of supply of sediment was greater than, or less than, the rate of increase of accommodation space available to be filled by sediment. For example, coastal sedimentation (of the type that may later yield hydrocarbon reservoir rocks) is generally into shallow water, where the sediment supply is more than sufficient to fill any available accommodation; on the other hand, slope sedimentation (of the type that may later yield hydrocarbon source rocks) is into deepening water, where the accommodation space is large and the sediment not already deposited to landward is not sufficient to fill it. In the prior art this distinction is sought in a study of fossil types, mineral inclusions, and the logs; particularly important is the gamma-ray log, which often allows a distinction (at least in part) on the basis of sediment grain size.
Major determinants of accommodation space are the subsidence of the depositional surface and variations of sea level. It is known that sea level often has a cyclic component; in effect, this alternately increases and decreases the accommodation made available by subsidence. Present knowledge of the variations of sea level through geologic time is sufficient to contribute to log interpretation, provided that the sea-level cycles (defined in time) can be recognized in the logs (defined in depth).
Also relevant to the background of the present invention is U.S. Pat. No. 5,999,485, to the present inventors. This patent discloses a statistic L offering a convenient measure of the thinness of rock layers. The statistic may be calculated simply and inexpensively, as a function of depth, from the logs.
Accordingly it is an object of the present invention to determine whether a particular portion of a log represents sediments deposited under conditions where sediment exceeded accommodation, or accommodation exceeded sediment.
It is a further object to associate a particular portion of a log, and the sediments it represents, with a phase of one or more sea-level cycles.
It is a further object to use a log, plus knowledge of sea-level variations through time, to construct a synthetic stratigraphic cross-section that is consistent with both, and that indicates the likely depositional environment of the sediments traversed by the log.
These objects are achieved in part by relating the thickness of a rock layer to the physical properties of the layer, using the following steps in combination
(a) measuring one or more appropriate physical properties of rock layers as a function of depth in the earth, where xe2x80x9cappropriatexe2x80x9d refers to those physical properties that display a cyclic aspect in the sense set out in U.S. Pat. No. 5,999,485;
(b) using the measurements of such properties to obtain an estimate of the rock layer thickness as a function of depth in the earth;
(c) using this estimate to determine the rate of change of the rock layer thickness as a function of depth in the earth (xe2x80x9crock-layer-thickness slopexe2x80x9d);
(d) using the physical-property measurements to determine the rate of change of each of the measured physical properties as a function of depth in the earth (xe2x80x9cphysical-property slopexe2x80x9d);
(e) identifying zones in the rock as first-type zones if within those zones sign of the rock-layer-thickness slope is the same as a physical-property slope and as second-type zones if these signs are opposite to one another, and
(f) ascribing to one type of these zones the characteristic of sediment exceeding accommodation and to the other type the characteristic of accommodation exceeding sediment.
Part (f) of the procedure described in the preceding paragraph is so phrased because the particular physical property selected for measurement and analysis determines which of the two types of zones defined is characterized by sediment exceeding accommodation and which by accommodation exceeding sediment.
The objects are further achieved by constructing a synthetic stratigraphic cross-section through a borehole, and thus determining the likely depositional environment of the stratigraphic intervals on the cross-section, using the following steps in combination, using as initial inputs trial values postulated for subsidence rate, sediment supply, sediment type, and equilibrium angle:
(a) identifying marker levels within the borehole, at each of which the geological age of deposition is known from biostratigraphical evidence;
(b) for each interval between such markers, using the interval thickness and a sea level cycle chart to obtain the local variation of sea level relative to the base of that interval, initially postulating that the observed subsidence rate was constant (the total subsidence within the interval being constrained at the end-points of the interval);
(c) postulating a cross-sectional form of a depositional substrate for the interval, which substrate may conform to the upper depositional surface constructed for one or more previous intervals;
(d) postulating a reasonable rate of supply of sediment, initially taking it to be constant during the chosen interval;
(e) postulating an initial range of a sediment property, such as grain size, likely to induce sorting during the processes of sediment transport and deposition;
(f) postulating an initial value for the equilibrium angle of a non-horizontal depositional surface for the sediment characteristics postulated above under conditions where accommodation exceeds sediment supply;
(g) using the method of the paragraph immediately above to identity those portions of the interval where sediment exceeded accommodation, and where accommodation exceeded sediment, respectively;
(h) using the inputs postulated in (b) through (f) above to construct one or more synthetic stratigraphic cross-sections of the rocks deposited in the chosen interval, such cross-sections being limited to those that are consistent with the actual data inputs from the borehole;
(i) by graphic or visual or other means, assessing the geological plausibility of such cross-section or cross-sections;
(j) perturbing each of the postulated inputs, iterating the calculation toward improved geological plausibility, finally selecting the synthetic stratigraphic cross-section deemed to be most geologically plausible.